1==================================
2RT-mutex subsystem with PI support
3==================================
4
5RT-mutexes with priority inheritance are used to support PI-futexes,
6which enable pthread_mutex_t priority inheritance attributes
7(PTHREAD_PRIO_INHERIT). [See Documentation/locking/pi-futex.rst for more details
8about PI-futexes.]
9
10This technology was developed in the -rt tree and streamlined for
11pthread_mutex support.
12
13Basic principles:
14-----------------
15
16RT-mutexes extend the semantics of simple mutexes by the priority
17inheritance protocol.
18
19A low priority owner of a rt-mutex inherits the priority of a higher
20priority waiter until the rt-mutex is released. If the temporarily
21boosted owner blocks on a rt-mutex itself it propagates the priority
22boosting to the owner of the other rt_mutex it gets blocked on. The
23priority boosting is immediately removed once the rt_mutex has been
24unlocked.
25
26This approach allows us to shorten the block of high-prio tasks on
27mutexes which protect shared resources. Priority inheritance is not a
28magic bullet for poorly designed applications, but it allows
29well-designed applications to use userspace locks in critical parts of
30an high priority thread, without losing determinism.
31
32The enqueueing of the waiters into the rtmutex waiter tree is done in
33priority order. For same priorities FIFO order is chosen. For each
34rtmutex, only the top priority waiter is enqueued into the owner's
35priority waiters tree. This tree too queues in priority order. Whenever
36the top priority waiter of a task changes (for example it timed out or
37got a signal), the priority of the owner task is readjusted. The
38priority enqueueing is handled by "pi_waiters".
39
40RT-mutexes are optimized for fastpath operations and have no internal
41locking overhead when locking an uncontended mutex or unlocking a mutex
42without waiters. The optimized fastpath operations require cmpxchg
43support. [If that is not available then the rt-mutex internal spinlock
44is used]
45
46The state of the rt-mutex is tracked via the owner field of the rt-mutex
47structure:
48
49lock->owner holds the task_struct pointer of the owner. Bit 0 is used to
50keep track of the "lock has waiters" state:
51
52 ============ ======= ================================================
53 owner        bit0    Notes
54 ============ ======= ================================================
55 NULL         0       lock is free (fast acquire possible)
56 NULL         1       lock is free and has waiters and the top waiter
57		      is going to take the lock [1]_
58 taskpointer  0       lock is held (fast release possible)
59 taskpointer  1       lock is held and has waiters [2]_
60 ============ ======= ================================================
61
62The fast atomic compare exchange based acquire and release is only
63possible when bit 0 of lock->owner is 0.
64
65.. [1] It also can be a transitional state when grabbing the lock
66       with ->wait_lock is held. To prevent any fast path cmpxchg to the lock,
67       we need to set the bit0 before looking at the lock, and the owner may
68       be NULL in this small time, hence this can be a transitional state.
69
70.. [2] There is a small time when bit 0 is set but there are no
71       waiters. This can happen when grabbing the lock in the slow path.
72       To prevent a cmpxchg of the owner releasing the lock, we need to
73       set this bit before looking at the lock.
74
75BTW, there is still technically a "Pending Owner", it's just not called
76that anymore. The pending owner happens to be the top_waiter of a lock
77that has no owner and has been woken up to grab the lock.
78